1. Field of the Invention
This invention relates generally to apparatus for detecting perforations in a traveling web and more particularly to a self-cleaning perforation detector and method.
2. Detection of the Prior Art
It is a well known practice to detect perforations in a traveling web. The perforations may be used to control web movement. Additionally the perforations may be used to control the operation of remote work stations or may be employed in retrieval of information from the web or controlling processing steps on the web as it travels about its course. Detection may be accomplished by directly sensing the perforations by mechanical means, or by indirectly sensing the perforations by electrical means such as optical or ultrasonic scanning mechanisms.
Typically, the perforation detector has a signal emitter on one side of the traveling web and a signal receiver for detecting the signal passing through the perforations or reflected from the web. When the web functions in an atmosphere laden with particulate matter (dust), or generates such particulate matter during web movement, the matter tends to accumulate on the signal emitter and/or the receiver. Such accumulation adversely affects the ability of the detector to accurately detect the perforations. Apparatus for cleaning the web may alleviate the problems associated with particulate matter generated by or adhering to the web surface, but does not prevent matter in the atmosphere from depositing on the signal emitter or receiver.